This disease, known as Crown Gall, crown knot, hairy-root, woolly knot, woolly-root and root gall, affects more particularly nursery stock. It is said that the Ben Davis, Early Harvest, Yellow Transparent, Wealthy, Grimes, Northern Spy, Oldenburg, Wolf River, Red June, Gano and Rome Beauty are most susceptible.
Although Crown Gall has been known in Europe for many years, it is the belief of certain authorities that the pathogene is native to this country. ' American nurserymen have known it for at least a half century. It is now found on practically every continent of the globe. Records come from Europe, South Africa, Asia, New Zealand and North America. It occurs in practically every state in the Union, but is most abundant in the more southern states of the apple - growing belt, from Virginia to Texas. The disease is also known in Canada.
As already intimated, Crown Gall is most damaging in the nursery, and becomes most prominent and important where root-grafts are carelessly made. In many instances the affected plant shows no ill effects, and trees are said to outgrow the disease if they are well rooted. In certain other cases affected plants are dwarfed, which of course renders them less valuable. A common source of loss lies in the fact that other patho-genes enter plants through the gall - lesions.
The disease may exhibit itself in two forms: (1) as galls (Fig. 30), and (2) as hairy-roots (Fig. 31). The galls are either hard or soft and occur at the ground-line or on limbs. It is estimated that fully 90 per cent of the galls appear, in the nursery, on the scion just above the point of union with the root. Mature galls may measure from one to several inches in diameter, are dark in color, with a roughened surface, and usually hard. Young galls are comparatively small, greenish, or sometimes slightly flesh-colored, relatively smooth, and soft or even spongy. The second form of the disease, hairy-root, is quite unlike the gall form just described. As the name suggests there is an excessive production of small fibrous roots, which give to the system a hairy appearance (Fig. 31). These may be grouped into several types, as follows: (a) a simple type, in which numerous small roots grow out at nearly right angles single or in tufts from an older root or stem; (b) a woolly form which originates from a smooth irregular swelling usually on the larger roots near the surface of the soil; (c) a broom - like formation in which there is excessive branching of the roots at the ends.
Fig. 30. - Crown Gall on apple roots.
Both the Crown Gall and hairy - root forms of the disease prevail in the nursery. The latter form is the more common in orchards.
Fig. 31. - Hairy - root of apple.
Extensive and reliable experiments have shown that this disease in its widely different forms is bacterial. The organism, Bacterium tumefaciens, has been known but a short time, and all points in its life - history are not fully understood.
It is unknown just where the bacteria hibernate, but most likely in the soil. They may live independently scattered through the soil, or in old galls which may persist from year to year. It is thought that the bacteria are carried from tree to tree: (1) by water of irrigation; (2) by cuttings from diseased plants; (3) probably by the pruning knife; and (4) by insects. The bacteria enter the host through wounds, and as a result of their rapid multiplication and stimulative action, the attacked plant shows signs of Crown Gall within a week or less. The plant tissue is invaded and the bacteria are found in the cells, but death does not result. On the other hand, the cells are stimulated to excessive multiplication. Soon after the first or primary gall is produced, particularly if the plant is rapidly growing, affected cells push out in strands, into the normal tissue, along the lines of least resistance. This effect is not visible externally, and is seen internally only with the aid of the microscope. The, presence of such a strand is evidenced by the fact that the removal of galls is not an effective control measure; new galls arise after the excision. As the strand of diseased cells proceeds, bacteria are carried to points, above and below the primary gall, where secondary galls arise. This manner of originating secondary galls represents a phenomenon similar to that exhibited in the case of malignant animal tumors or cancers; because of this similarity the disease is sometimes referred to as plant - cancer.
It is understood then that infection may arise internally and externally. The bacteria may come not only from the apple, but from many other plants, including the almond, apricot, beet, blackberry, cherry, chestnut, clover, cotton, daisy, English walnut, grape, honeysuckle, hop, lettuce, peach, plum, poplar, potato, prune, quince, rose, tobacco, tomato, turnip and willow. This list is incomplete but will give an idea of the wide range of plants susceptible to the disease, and therefore of the many sources from which the bacteria may come to the apple.
In dealing with Crown Gall, the following points should be borne in mind: (1) the great number of plants attacked; (2) the identity of the bacteria causing hairy-root and Crown Gall; (3) the greater abundance of the disease in nursery-stock; (4) the bacterial nature of the disease; (5) the entrance of the pathogene into the host through wounds; (6) its presence in the soil; (7) the distribution of the pathogene in nursery-stock; (8) internal infection by tumor strands, and that therefore not all galls necessarily arise from an external source. In the light of these data the following measures and precautions should be employed: (1) deep planting to protect the plant against Frost Injury; (2) avoid injuries to the roots and stems; (3) remove diseased trees; gall excision is unsatisfactory and unreliable; (4) protect grafts at the point of union by using a root and scion of approximately the same size, and by wrapping them carefully; (5) reject diseased nursery-stock. (See Table of Contents for other fruits which are affected by Crown Gall.)
Smith, E. F., Brown, N. A., and Townsend, C. O. Crown Gall of plants: its cause and remedy. U. S. Agr. Dept. Plant Indus.
Bur. Bul. 213: 3 - 215. 1911. Smith, E. F., Brown, N. A., and McCulloch, L. The structure and development of crown gall: a plant cancer. U. S. Agr. Dept.
Plant Indus. Bur. Bul. 255: 3-60. 1912. Swingle, D. B. Fruit diseases in Montana. Crown Gall of apple.
Montana Agr. Exp. Sta. Circ. 37: 290 - 301. 1914.
Hedgcock, G. G. Field studies of the Crown Gall and hairy root of the apple-tree. U. S. Agr. Dept. Plant Indus. Bur. Bui. 186: 9 - 96. 1910.
Cook, M. T. Crown gall and hairy root. New Jersey Agr. Exp. Sta. Circ. 34: 3 - 14. 1914.